A reason has now been given for why Tumblr’s iOS app has been removed from Apple’s App Store. The social networking service said that child sexual abuse content had been found on the platform during a routine audit. The app has been missing from the store since November 16th, but until now the reason for its absence was unclear.

Apple requires iOS apps to have content filters to prevent the distribution of child pornography. The App Store guidelines say, “Apps with user-generated content or services that end up being used primarily for pornographic content, objectification of real people (e.g. “hot-or-not” voting), making physical threats, or bullying do not belong on the App Store and may be removed without notice.” There has been no timeline given for when the app may be returned to the App Store.

Tumblr does use an “industry database” as a content filter to prevent the distribution of prohibited content. Tumblr explained, “Every image uploaded to Tumblr is scanned against an industry database of known child sexual abuse material, and images that are detected never reach the platform. A routine audit discovered content on our platform that had not yet been included in the industry database.”

Tumblr has always had a permissive attitude towards adult content, but has taken a strong stance against child pornography. Tumblr’s statement said, “We’re committed to helping build a safe online environment for all users, and we have a zero tolerance policy when it comes to media featuring child sexual exploitation and abuse … We immediately removed this content.”

The company’s statement continued, “Content safeguards are a challenging aspect of operating scaled platforms. We’re continuously assessing further steps we can take to improve and there is no higher priority for our team.” The app is still available in the Google Play store for Android users.